1. Field of the Invention
The present invention relates generally to a pulse width modulated (PWM) signal amplifier and is directed more particularly to a PWM signal amplifier with which longitudinal amplitude distortion is reduced.
2. Description of the Prior Art
Recently, pulse width modulated signal amplifiers have been used as audio output amplifiers in class D amplifiers. In such cases, since the PWM signal amplifier can perform a complete switching operation from a theoretical point of view, power consumption by the switching element is very small and hence the efficiency of the amplifier is improved.
There are circuits used in PWM amplifiers in which two transistors are connected in a complementary push-pull arrangement in series between the terminals of a DC power source. The connection point between the transistors is connected through a low pass filter, such as a choke coil, to a speaker. In such circuits, the discharging current of the choke coil is partially by-passed through diodes connected in parallel to the transistors to carry out a predetermined switching operation.
With the type of prior art circuit just described, when the transistors and diodes are biased to be conductive and nonconductive alternately, a type of amplitude distortion known as longitudinal amplitude distortion is produced in the waveform of the output pulse voltage. Such a defect is very disadvantageous for reducing the distortion of a class D amplifier.